One problem many people working out forget is the necessity for change. I mean if you do the same thing over and over, you will get the same old results.
One thing that is very effective is simply changing rep speed. I do a lot of this with my clients, and there is scientific research to back up this technique. A few years back, Nautilus did a study using a 4 second positive and 10 second negative rep speed. The end result was that having a muscle under tension for 70-90 seconds yielded better results than faster rep speeds. The problem however, is that working out this way can be painful causing people to want to "go back to the old way."
Still, varying speeds can be used. I often have my clients use a simple 4 second up and 4 second down speed. This is an effective change that can be thrown into the mix to keep the body guessing. Don't be shy about change. Using a six and two count is another great routine. Use your imagination! Change is what will get your body to grow.
My last comment is a warning. If you are using slower rep speeds, you will likely have to reduce a little of the weight or resistance you are using. The increased time will certainly make things seem harder.
Change things up to change your body!